This invention relates to control systems for auxiliary locomotion drive systems of a vehicle. More specifically this invention relates to a speed control system for the front wheel assisting drive of a vehicle having a main rear wheel powered drive. Many vehicles are typically driven by applying drive power to the rear wheels thereof. Some vehicles are provided with an auxiliary drive system connected to the front wheels thereof for use during low speed operation of the vehicle powering the front wheels to assist where tractive conditions are either poor or variable and the rear wheels are likely to slip.
A vehicle is a construction machine such as a motor grader. Powering of the front wheels is a benefit when the machine is operating on a slippery surface such as sand, mud, snow or ice, particularly during low speed operations of the machine. When such a machine is operated at a low speed, if the front wheels rotate at the same rate as the rear wheels, a continuous assisting action is provided, thus changing the driving and operating characteristics of the vehicle from that which an operator would experience for a normally only rear wheel driven vehicle.
For operation of a vehicle, such as a motor grader, in a high speed mode of operation, such as between 10 and 25 m.p.h. for high speed grading, snow removal and general transport between two locations, the front wheel assistance is not necessary. During such operation, if a front wheel powering system is in operation, it must be capable of operating at this speed, which is substantially higher than the speed at which it provides a useful drive assistance. For motor graders that have mechanical rear wheel drive and would incorporate a hydraulically assisted front wheel drive, operation at a high speed can present overheating problems for the hydraulic system serving the assisting front wheel. Overheating would be due to forced operation of the hydraulic system beyond it's normal expectant capacity. More specifically, the hydraulic drive system is designed to provide rotation of the front wheel up to a speed of about 12 m.p.h. Above that speed it has been found that continued operation of the assisting front wheel drive is not necessary because grading with the vehicle and maneuvering are done at speeds below that and it is those speeds at which the driving assistance of the front wheel is needed. The hydraulic circuit is designed for operation of a speed of about 12 m.p.h. or below and operation above that speed exceedingly stresses the system, thus raising the potential for failure of operation.